1. Field of the Invention
The present invention is generally concerned with protecting a circuit of any kind against overcurrents such as those caused by a short circuit, for example.
It is more particularly directed to the situation in which instead of using a fuse of any kind to provide this protection, in the conventional way, where the fuse must be changed each time it operates, the current is read, or to be more precise, the current level is detected, for appropriate intervention in the circuit to be protected as soon as the current level reaches a particular threshold.
2. Description of the Prior Art
The current level detector devices usually employed use resistor means inserted into the circuit to be protected and serving as a sensor and a detector cell shunting the resistor means.
In alternating current circuits there are two feasible solutions.
A first solution uses only one detector cell and rectification with no threshold is therefore employed.
However, this solution has the disadvantage of requiring considerable and costly post-processing of the information collected.
A second solution uses two detector cells, one for each half-wave of the current, and each of the two detector cells is connected to specific resistor means in a mirror type circuit.
This has the advantage of simplifying the required processing of the information collected.
In practice the two detector cells employed are similar and each has two inputs connected to the corresponding resistor means.
The resistor means can in some cases be a low value resistor and in this case heat dissipation within the resistor advantageously remains moderate.
This is not always the case, however, especially if the associated detector cell uses a selectively operable switch, for example a transistor, which is triggered by applying a sufficient potential to its base circuit.
Heat dissipation in the resistor means can remain acceptable if the power in the circuit to be protected remains moderate, however.
This is not the case for higher powers, for example if the circuit to be protected uses power semiconductors.
A general aim of the present invention is an arrangement providing a very simple and effective way to reduce heat dissipation in this case, and which also has other advantages.